The present invention is directed to an interface for coupling a transmitter to an optical fiber, and more specifically to a tertiary optical media interface for providing information to an optical superposition network that utilizes a time division multiplexing (TDM) protocol.
As is well known to one of ordinary skill in the art, a superposition network includes any network in which various signals combine additively on a network media. The TDM protocol, which has been widely utilized with various networks that share a common communication path (e.g., Ethernet), is also well known to one of ordinary skill in the art. In a network that implements a TDM protocol, all nodes wishing to communicate over the network take turns such that only one node transmits at a time. This has typically been accomplished with either a collision avoidance scheme (i.e., where a frame is assigned a unit of time, which is divided into time slots with each node being assigned a time slot or slots within which to transmit in each frame) or a collision resolution scheme (e.g., carrier sense multiple access/collision detection (CSMA/CD)). Some electrical networks that have utilized a TDM protocol have also used tertiary signaling for communication.
In a network that implements a TDM protocol, each receiver may receive data from multiple transmitters. This data is a time interleave of the transmitted signals from the multiple transmitters. Due to the variation in transmitter power and path attenuation, the time multiplexed signals produced by the multiple transmitters frequently have different amplitudes at a given receiver. As a result, each receiver coupled to such a network has been required to include the capability of compensating for the signal variation.
In electrical networks, when the low frequency properties of a received signal are known (or controlled through, for example, coding), an appropriate threshold can be derived from the received signal (by low pass filtering the received signal). For example, bi-phase coding (where each bit of data is represented by two bits) provides a threshold, which is midway between a minimum and a maximum of a signal. While bi-phase coding is desirable for clock synchronization, it undesirably reduces the network bandwidth by approximately one-half. In the situation where the appropriate threshold is the average of the maximum and the minimum signal levels, an AC coupled receiver (referenced to a common ground) can be utilized without signal distortion. While utilizing such a receiver in an optical superposition network, which utilizes binary signaling with varying received amplitudes, yields received signals that have a constant base line (the common ground), the received signals may have a non-constant center line (desired threshold).
One approach to resolving the problem of a non-constant center line (threshold) has been to use a receiver with an adaptive threshold. One technique has tracked a minimum and a maximum level of a received signal and set a threshold midway between the two. Unfortunately, this technique is susceptible to noise. Another technique has adjusted the threshold for each transmitter. However, this technique requires a transmission preamble from which the threshold can be determined, which undesirably reduces the bandwidth of the network.
Optical networks, currently in use, have primarily utilized a multiple point-to-point topology. In optical networks that use a point-to-point topology, each node typically includes a different receiver for each node from which it receives signals. As such, a threshold of each different receiver can be individually adjusted to compensate for the received signal amplitude. Another topology that has seen limited use in optical networks is the star topology. The optical star topology includes a central hub and a number of nodes, which communicate through the hub via optical fibers. In a passive optical star network, the hub functions to combine and then split the light signals it receives. In a passive optical star network, a receiver within a single node may be coupled to multiple transmitters that provide signals of different intensities.
In current optical networks, the optical transmitting device (e.g., light emitting diode (LED)) has been a binary element. That is, the LED has either been on or off. At the receiver, the received signal has been compared to a threshold, which has been set between a light level and a dark level. Above the threshold, the received signal is considered light or a digital xe2x80x9c1xe2x80x9d. Below the threshold, the received signal is considered dark or a digital xe2x80x9c0xe2x80x9d. However, because the intensity of the received signal may not be known in advance, the threshold has not typically been fixed. That is, the threshold has been a function of the received signal intensity. As such, optical networks that include receivers that are coupled to multiple transmitters (that may provide signals of varying intensities) have been required to include the capability of compensating for this received signal variation.
Thus, the development of a technique which allows multiple transmitters coupled to an optical superposition network to provide signals that have an approximately constant average intensity (threshold) is desirable.
The present invention is directed to a technique for electronically transmitting information across an optical superposition network that utilizes a time division multiplexing (TDM) protocol. In such a network, a first optical transmitter, a second optical transmitter and a receiver are coupled to an optical fiber. The first transmitter transmits information in a tertiary mode during a time slot. The second transmitter transmits information in a tertiary mode during a different time slot. The information transmitted across the optical fiber has an approximately constant average intensity such that the receiver can utilize a single decision threshold for the information received from both the first and second optical transmitters.